In electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is well known and useful for light lens copying from an original and printing applications from electronically generated or stored originals, where a charged surface may be imagewise discharged in a variety of ways. Ion projection devices where a charge is imagewise deposited on a charge retentive substrate operate similarly. While heretofore, toners selected for development have been primarily black, colored toners, for producing full or partial color, highlight color or single non-black color images are becoming an important feature of current copier products.
One important process control in electrophotographic applications is the measurement of the amount of unfused toner that is developed onto the charge retentive surface. In Infrared Reflection Densitometer (IRD) sensors, such as that used for example in the Xerox 1065 copier, heretofore primarily for use with black toners, at a test patch exposed and developed specifically for process control purposes, a light emitting infrared diode illuminates the toner/surface combination, and a photodiode is positioned to collect specularly reflected light. Black toner is light absorbing, so that as more toner is developed onto the surface, the amount of light specularly reflected from the toner/surface combination is attenuated. The process control measurement is the ratio between light reflected from the toner/surface combination and the bare surface. This ratio is referred to as the specular reflection ratio.
It has been found that when specular reflection-based developability sensors are used with colored toners, the specular reflection ratio becomes invariant for DMA (developed toner mass per unit area) values smaller than those associated with optimal control values. For example, it has been noted that when using colored toners with a mean diameter of 9 microns, the specular reflection ratio becomes insensitive to further changes in DMA for DMA values between 0.5 and 0.6 mg/cm.sup.2. A desirable DMA sensing range for a colored toner with a mean diameter of 9 microns would extend toward 1.0 mg/cm.sup.2. This decrease in sensitivity appears to result from scattering of light by colored toners, since colored toners tend to be poor absorbers of infrared light. It should be also noted that the specular reflection ratio is invariant with a large value compared to zero, as a result of the large angle subtended by the detector, and required to provide latitude in mounting the sensor with respect to the surface. Thus, for colored toners, both specular and diffusely reflected light is detected by the sensor. The diffusely reflected light tends to obscure changes in the specular component, and gives signal saturation at a substantial offset value.
U.S. Pat. No. 4,553,033 to Hubble, III et al. teaches an LCIRD of the type useful for detecting the specular component of reflected light. U.S. Pat. No. 3,801,349 to Wilson et al. teaches a coating density sensor that measures the density of light absorbing powder on a diffusely reflective substrate at a location, preferably at the specularly reflective location. U.S. Pat. No. 2,956,487 to Gaimo, Jr. which, while unclear as to the type of reflected light, appears to be detecting specularly reflected light. U.S. Pat. No. 3,094,049 to Snelling measures toner concentration by measuring transmission of light through toner. U.S. Pat. No. 3,778,146 to Knapp measures toner concentration by measuring transmission of light through toner. U.S. Pat. No. 3,801,196 to Knapp et al. measures toner concentration by measuring light reflected from a reflecting arrangement on a photoconductor, which provides a double pass transmission measurement, at what appears to be a specular reflection angle. U.S. Pat. No. 4,082,445 to Steiner appears to measure reflected light at a test patch for comparison to a clean patch on the photoreceptor. U.S. Pat. No. 4,026,643 to Bergman combines a clean patch/test patch measurement with a measurement of photoreceptor potential with and without charged particle development. U.S. Pat. No. 4,178,095 to Champion et al., U.S. Pat. No. 4,179,213 to Queener, and U.S. Pat. No. 4,183,657 to Ernst et al. all appear to show various aspects of a sensor for the specular component of reflection. U.S. Pat. No. 4,054,391 to Witte discloses a specular reflectance microdensitometer wherein the amount of light specularly reflected by a test surface is correlated to the density of particle coverage on the surface. The reflectance of a clean photoreceptor surface and a toned photoreceptor surface is measured and the ratio determined. U.S. Pat. No. 4,284,356 to Heilman describes light sources and light detectors for illuminating and comparing surface reflectivity.
U.S. patent application Ser. No. 246,242 filed Sept. 19, 1988 and assigned to the same assignee as the present application describes apparatus projecting light rays onto the particles on a surface. Total reflectivity of at least the particles and the diffuse component of the total reflectivity of at least the particles is detected, and a total signal indicative of the total reflectivity of at least the particles and a diffuse signal indicative of the diffuse component of the total reflectivity of at least the particles. The difference between the total signal and the diffuse signal is measured and a signal is generated indicative of the specular component of the total reflectivity of at least the particles.
Certain toners that give the appearance of black toner may be composed of a combination of colored pigments mixed to present a black appearance, rather than carbon black pigments. It is of interest that such toners have retained the light absorbing characteristics of colored toners even though they appear black.